1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplex Access (OFDMA) system, and more particularly, to a downlink beamforming apparatus and a transmission apparatus including the same.
2. Description of the Related Art
An OFDMA system needs to adopt transmission techniques for beamforming and diversity using multiple antennas in order to improve performance and capacity of the OFDMA system.
Beamforming and diversity are defined in the wireless access standard, such as IEEE 802.16e Wireless Local Area Network (WLAN) OFDMA PHY, in order to extend cell coverage by improving a link budget of the OFDMA system because the Adaptive Array System (AAS) technique is applied to a system including one terminal antenna.
An uplink channel sounding signal is defined in a transmitting apparatus of an OFDMA/Time Division Duplex (TDD) system applying beamforming transmission in order for a base station to measure and convert measured uplink channel information into prospective downlink channel reaction. In addition, a terminal periodically transmits a sounding signal in order to support prediction of a downlink channel under a mobile environment.
A base station can measure uplink channel information by using uplink channel sounding information and reciprocity of a TDD system, and obtain a weigh value for downlink beamforming by using the uplink channel information so as to form a downlink beam pattern. An apparatus for transmitting beamforming applied to an OFDM TDMA system in IEEE 802.11a is described in US Patent Publication No. 2005/0078763, entitled “Apparatus and Method of Multiple Antenna Transmitter Beamforming of High Data Rate Wideband Packetized Wireless Communication Signals”.
Although many users of the OFDMA system simultaneously share subcarriers used for the OFDMA system, uplink/downlink users for the same frame may not always be scheduled identically. Accordingly, uplink channel information needs to be detected by each user and each antenna, and then stored at a buffer in order to be applied to a downlink transmit frame that is consecutive to the uplink channel information. When the uplink channel information is directly applied to a downlink, a weight value may be selectively multiplied by each user because a downlink transmitter allocates subcarriers by each user according to the OFDMA system. However, when an uplink frame receives information for Nu users, a beamforming weight value needs to be stored at a buffer of which a size is as much as Nu×K×Na×2×Nused in order for the information received by the uplink frame to be applied to the downlink frame consecutive to the uplink frame. Na denotes the number of antennas of the base station, 2 denotes I/Q (In phase/Quadrature phase) channel, Nused denotes the number of subcarriers used for one symbol of OFDM system, and K denotes word length representing channel information.
However, since K is generally a large amount, hardware complexity of a beamforming apparatus undesirably tends to increase.